Spin defects in solid state systems, such as the NV (nitrogen-vacancy) defect centre in diamond, have numerous potential applications. These applications include, without limitation, nanoscale electric and magnetic-field sensing, single-photon microscopy, quantum information processing, and bioimaging.
NV-centre based nanosensors rely on the ability to position a single nitrogen-vacancy centre within a few nanometers of a sample, and then scan it across the sample surface, while preserving the NV centre's spin coherence and readout fidelity.
Existing scanning techniques, however, suffer from drawbacks that include low sensitivity, low resolution, and high data acquisition times. It is considered that these drawbacks are due to a number of factors including one or more of: short spin coherence times due to poor crystal quality; too large a distance between the spin defect and the sample surface being analyzed; variations in the distance between the spin defect and the sample surface being analyzed; and inefficient far-field collection of the fluorescence from the NV centre.
For example, one known technique utilizes a diamond nano-particle containing an NV spin defect. The diamond nanoparticle is adhered to an optical fiber to optically address the NV defect within the diamond nano-particle, a microwave generator is utilized to manipulate the spin state of the NV defect when the diamond nano-particle is placed in close proximity to a sample to be analyzed, and a detector is provided on an opposite side of the sample to detect fluorescence from the NV defect.
The aforementioned configuration has a number of problems. First, while the use of a diamond nano-particle ensures that the NV defect can be positioned close to the sample to be analyzed, diamond nano-particles tend to be of poor diamond quality and the NV defects therein have short spin coherence times and can be optically unstable leading to poor sensitivity. Secondly, fluorescent light is emitted in all directions and only a small proportion can be detected. Thirdly, the detector is disposed on an opposite side of the sample to the diamond nano-particle and thus the configuration can only be used for material samples which are transparent to the fluorescent emission. While the optical detector could be positioned on the same side of the sample as the diamond nano-particle, it is difficult to arrange the detector to effectively capture fluorescent emission because the diamond nano-particle is adhered to the end of an optical fiber which inhibits detection of fluorescence on the same side of the nano-particle as the optical fiber.
An alternative to the aforementioned configuration would be to use a high quality single crystal diamond material comprising an NV defect which has a longer spin coherence time. However, the use of a micron scale single crystal diamond material has a number of problems including, for example: too large a distance between the spin defect and the sample surface being analyzed; variations in the distance between the spin defect and the sample surface being analyzed; and inefficient far-field collection of the fluorescence from the NV centre.
It is an aim of certain embodiments of the present invention to solve one or more of the aforementioned problems.